1. Field of the Invention
The present invention relates to a transparent electrode substrate for use in transparent electrode plates and the like of devices such as liquid crystal display elements and solar cells, and in particular to a transparent electrode substrate with increased heat resistance.
2. Description of the Related Art
Transparent electrode plates of liquid crystal display elements and solar cells and the like typically use a transparent substrate such as glass, on which is provided a thin film of thickness 50 to 1,000 nm of indium oxide doped with several percent of tin, known as an indium tin oxide film (hereafter abbreviated as an ITO film).
However, although this ITO film has excellent transparency and a high level of conductivity, it suffers from deteriorating heat resistance at temperatures over 300° C.
Namely, when ITO films are exposed to a high temperature of 300° C. or higher, the electrical resistance of the ITO film increases more than three times, and the conductivity decreases markedly. It is thought that the reason for this behavior that on heating, oxygen from the atmosphere bonds to a portion of the oxygen vacant structures within the ITO film, reducing the oxygen vacancies which functions as a passage for electrons.
Consequently, if this type of transparent electrode plate with an ITO film formed thereon is used, for example, to make a dye sensitized solar cell, then a paste formed from fine oxide powder such as titanium oxide is coated on the surface of the ITO film of the transparent electrode plate, and the paste is calcinated at a temperature of 400 to 600° C. to form porous oxide semiconductor film. However, during this process, the conductivity of the ITO film of the transparent electrode plate deteriorates markedly. As a result the photoelectric conversion efficiency of the dye sensitized solar cell also deteriorates.
For example, Japanese Unexamined Patent Applications, First Publication Nos. Hei 7-249316 and Hei 9-063954 disclose the conventional transparent electrode plate.
Accordingly, an object of the present invention is to provide a transparent electrode substrate with an ITO film formed thereon, used for example as the transparent electrode plate in a dye sensitized solar cell, for which the electrical resistance does not increase even when exposed to high temperatures of 300° C. or higher.